Anisotropic longitudinal water proton relaxation in white matter investigated ex vivo in porcine spinal cord with sample rotation

Abstract

A variation of the longitudinal relaxation time $T_1$ in brain regions that differ in their main fiber direction has been occasionally reported, however, with inconsistent results. Goal of the present study was to clarify such inconsistencies, and the origin of potential $T_1$ orientation dependence, by applying direct sample rotation and comparing the results from different approaches to measure $T_1$ . A section of fixed porcine spinal cord white matter was investigated at 3 T with variation of the fiber-to-field angle ${\theta }_{\text{FB}}$ . The experiments included one-dimensional inversion-recovery, MP2RAGE, and variable flip-angle $T_1$ measurements at 22 °C and 36 °C as well as magnetization-transfer (MT) and diffusion-weighted acquisitions. Depending on the technique, different degrees of $T_1$ anisotropy (between 2 and 10%) were observed as well as different dependencies on ${\theta }_{\text{FB}}$ (monotonic variation or $T_1$ maximum at 30-40°). More pronounced anisotropy was obtained with techniques that are more sensitive to MT effects. Furthermore, strong correlations of ${\theta }_{\text{FB}}$ -dependent MT saturation and $T_1$ were found. A comprehensive analysis based on the binary spin-bath model for MT revealed an interplay of several orientation-dependent parameters, including the transverse relaxation times of the macromolecular and the water pool as well as the longitudinal relaxation time of the macromolecular pool.

Keywords: Anisotropy; Inversion recovery; Longitudinal relaxation; MP2RAGE; Magnetization transfer; Orientation dependence.